Many liquids, such as alcoholic beverages, fruit juices and dairy products rapidly deteriorate when exposed to atmospheric conditions and more specifically, when exposed to oxygen following the initial opening of the container. One particular product that is particularly susceptible to deterioration due to rapid oxidation is wine due to the basic chemistry of wine. For most wines, winemakers go to great lengths to protect them from seeing too much oxygen. Deciding just how much is too much, both during winemaking and for bottled wine, is at the heart of many of the wine world's most heated current debates. The precise role of oxygen in wine development and ageing is still being unraveled. To prevent oxidation, bottling practices are followed by most commercial wineries that minimize the exposure to oxygen. This precaution is further complicated by the use of closures, such as natural cork, that are susceptible to oxygen intrusion or other contaminants. Despite all the measures taken before and during the filling process, when the container is subsequently opened, oxygen intrusion immediately occurs and the process of oxidation and spoilage begins resulting in the remaining product rapidly degrades and spoils.
While some wine shortcomings are more difficult to experience, oxidation is much easier to experience. Simply take a bottle of wine, pour a couple of glasses and enjoy and then recork the bottle and leave it on your counter for a few days. After a few days, pour another glass and compare your impressions of this glass of wine, which will by now be partly oxidized, with your previous experience of the wine from the same bottle. No doubt, your experience will be less than satisfactory. With oxidation, it's not so much what it contributes as what it takes away. The dominant feature one experiences is one of flatness. This is because exposure to oxygen has taken out some of the volatile chemicals that are an important part of wine aroma. However there is also a contribution from chemicals formed by the oxidation process, the most important of which is acetaldehyde, and in addition, the fruit in oxidized red wines begins to take on a caramel-like quality, and oxidized whites wines become heavy and dull. Moreover, the palate of oxidized red wines also changes since the wines tend to take on a dry, slightly bitter characteristic. In addition, oxidation causes color changes in the wines.
Because the interaction of oxygen and wine is potentially damaging, wine needs to be protected both during the bottling process and after opening the bottle if the entire bottle is not consumed during one sitting. There have been a number of attempts to minimize the effects of oxidation on the liquid that is within the container; however, each of these attempts has its own shortcomings.